The hack was first discovered when two computers infected with
malicious software tried to contact a server in Eastern Europe to
transfer stolen data. IT staff at the Bundestag noticed the
attempt to smuggle out the data, but realised that other
computers had also been compromised.

Now the finger of blame for the hack is pointing at Russia. The
Register reports that investigators have managed to read the
source code used to hack into the German parliament, and it looks
similar to an attack in 2014 on a German data network that was
traced back to Russia.